Process for liquefying a hydrocarbon-rich fraction

ABSTRACT

Described herein is a process for liquefying a hydrocarbon-rich fraction, in particular natural gas, is described, in which
         the hydrocarbon-rich fraction that is to be liquefied is cooled in indirect heat exchange against a multistage precooling circuit,   the refrigerant of the precooling circuit is at least 95% by volume carbon dioxide,   the cooled hydrocarbon-rich fraction is liquefied and subcooled in indirect heat exchange against a mixed cycle, and   the mixed refrigerant of the mixed cycle comprises exclusively the component(s) nitrogen, methane and/or ethane.

FIELD OF THE INVENTION

The invention relates to a process for liquefying a hydrocarbon-rich fraction, in particular natural gas.

Classical natural gas liquefaction systems of the capacity range from 1 to 5 million tons per annum (mtpa) of LNG (Liquefied Natural Gas) are predominantly based on a process having a propane precooling and a mixed cycle for liquefaction and subcooling of the natural gas. Such a liquefaction process is described, for example, in U.S. Pat. No. 3,763,658 (Gaumer et al.).

It is additionally known to use carbon dioxide as refrigerant for precooling in natural gas liquefaction. The lowest easily reachable temperature, however, is limited by the triple point of carbon dioxide to about −56° C., since below this temperature carbon dioxide is present in solid form which makes a continuous process procedure difficult.

The previously described liquefaction process which is outstandingly suitable for land systems uses a refrigerant containing higher hydrocarbons, in particular propane, not only in the precooling of the pure substance but also in the mixed cycle. These higher hydrocarbons, in the event of leaks in the system, form gas clouds having a density greater than air. As a result, under some circumstances, hazardous, explosive air-hydrocarbon mixtures can form, which are categorized as a considerable safety hazard.

In the case of floating natural gas liquefaction systems (FLNG), therefore, attempts are made to avoid flammable refrigerant entirely—for example, by using nitrogen, carbon dioxide or HCFCs—or to limit the spread of local system faults by suitable safety distances between potential hazardous sources. In all cases, however, the space requirement for the liquefaction system increases, and therefore also the capital costs owing to the required enlargement of the relatively expensive ship's hull. This is due, for example, in the case of what are termed N₂ expander processes, to the relatively low thermodynamic efficiency, which requires larger compressors and drives for a given liquefaction output.

Therefore, an object of the present invention is to provide a process of the type in question for liquefying a hydrocarbon-rich fraction which, owing to a more compact structure, has a smaller space requirement compared with the liquefaction processes currently realized in FLNG systems. In addition, the process should satisfy the prescribed safety requirements.

Upon further study of the specification and appended claims, other objects, aspects and advantages of the invention will become apparent.

SUMMARY OF THE INVENTION

To achieve these objects, a process for liquefying a hydrocarbon-rich fraction is proposed, in which

-   -   the hydrocarbon-rich fraction that is to be liquefied is cooled         in indirect heat exchange against a multistage precooling         circuit,     -   the refrigerant of the precooling circuit is at least 95% by         volume carbon dioxide,     -   the cooled hydrocarbon-rich fraction is liquefied and subcooled         in indirect heat exchange against a mixed cycle, and     -   the mixed refrigerant of the mixed cycle comprises exclusively         component(s) selected from nitrogen, methane and ethane.

The process according to the invention for liquefying a hydrocarbon-rich fraction is distinguished in that a multistage single-component precooling circuit and a mixed cycle which serves for liquefying and subcooling the hydrocarbon-rich fraction are combined, wherein the refrigerant of the precooling circuit is at least 95% by volume, preferably at least 99% by volume, carbon dioxide, while the mixed refrigerant of the mixed cycle comprises exclusively the component(s) selected from nitrogen, methane and ethane (e.g., comprises 5-30 mol % nitrogen, 20-60 mol % methane and 20-60 mol % ethane).

The use of higher hydrocarbons—these are taken to mean C₃₊-hydrocarbons—is dispensed with completely. Using the procedure according to the invention, FLNG systems can—without having to accept reductions in safety—be implemented in a more compact and thus cheaper form.

Compared with optimized liquefaction processes which are customarily used in land-based systems, the process according to the invention for liquefying a hydrocarbon-rich fraction has a higher energy consumption. The excess energy consumption is approximately 7%, in unfavorable applications, a maximum of 10%.

Further advantageous embodiments of the process according to the invention for liquefying a hydrocarbon-rich fraction are characterized in that

-   -   the carbon dioxide circulating in the precooling circuit is         compressed in two separate compressor casings, wherein the exit         pressure of the low-pressure casing is below the critical         pressure of carbon dioxide,     -   the exit pressure of the high-pressure casing is operated at a         final pressure of at least 90 bar, preferably at least 100 bar,     -   the mixed refrigerant circulating in the mixed cycle is         compressed to a pressure above the critical pressure thereof,     -   the temperature(s) of the hydrocarbon-rich fraction that is to         be liquefied, of the carbon dioxide and/or of the mixed         refrigerant is/are adjusted in such a manner that the drive         powers of the compressors of the precooling circuit and of the         compressor or compressors of the mixed cycle differ by a maximum         of 10%, preferably by a maximum of 5%,     -   if the hydrocarbon-rich fraction that is to be liquefied, before         the cooling and liquefaction thereof, is subjected to an         adsorptive water removal, the liquefied hydrocarbon-rich         fraction is expanded, the resulting gaseous fraction is         compressed to the pressure of the hydrocarbon-rich fraction that         is to be liquefied and fed thereto, wherein the compressed         gaseous fraction, before the feeding thereof, serves for         regenerating the dryer or dryers of the adsorptive water         removal,     -   the refrigerant of the precooling circuit is at least 99% by         volume carbon dioxide, and     -   higher hydrocarbons and optionally hydrocarbons at risk of         freezing, such as, for example, benzene, are removed from the         precooled hydrocarbon-rich fraction that is to be liquefied.

BRIEF DESCRIPTION OF THE DRAWINGS

The process according to the invention for liquefying a hydrocarbon-rich fraction and further advantageous embodiments of the same will be described in more detail hereinafter with reference to the exemplary embodiments shown in FIGS. 1 and 2.

In the embodiment shown in FIG. 1 of the process according to the invention, the hydrocarbon-rich fraction A that is to be liquefied is first fed to an amine scrubber X and therein freed from components, in particular carbon dioxide and sulphur compounds, which interfere in the subsequent liquefaction. Then, the hydrocarbon-rich fraction A′ is cooled in the heat exchanger E1C, which will be considered in more detail hereinafter, and then fed to an adsorptive water removal unit Y.

The hydrocarbon-rich fraction B pretreated in this manner is then cooled in the heat exchangers E1B to E4B against the refrigerant of the precooling circuit, which will be considered in more detail hereinafter. In the heat exchanger E7/E8, the cooled hydrocarbon-rich fraction C is liquefied (heat exchanger section E7) and subcooled (heat exchanger section E8) in indirect heat exchange against the mixed refrigerant of the mixed cycle, which will be considered in more detail hereinafter.

Optionally, before entering the heat exchanger E7/E8, higher hydrocarbons and optionally hydrocarbons at risk of freezing, such as, for example, benzene, are removed from the precooled hydrocarbon-rich fraction that is to be liquefied in removal unit Z. At the cold end of the heat exchanger E7/E8, the liquefied and subcooled hydrocarbon-rich fraction D is withdrawn and fed, for example, to an atmospheric storage tank S. For this purpose, the hydrocarbon-rich fraction D is expanded in valve V2 to the desired storage pressure. The resultant gaseous fraction E can, according to an advantageous embodiment of the process according to the invention, be compressed in compressor C3 to the pressure of the hydrocarbon-rich fraction A that is to be liquefied, and fed thereto, wherein the compressed gaseous fraction, before the feeding thereof, preferably serves for regenerating the dryer or dryers of the adsorptive water removal unit Y.

The previously mentioned precooling circuit in which carbon dioxide circulates according to the invention as refrigerant, in the embodiment shown in FIG. 1, has a compressor unit which consists of two separate compressor casings C1A and C1B. In this case the carbon dioxide streams 6, 7 and 8 that are fed to the low-pressure casing C1A are preferably only compressed to a pressure which is below the critical pressure of carbon dioxide. The carbon dioxide 9 compressed in this manner is cooled in the aftercooler E5A against a suitable medium and fed to the high-pressure casing C1B of the compressor unit. Therein, the carbon dioxide is compressed to the desired final pressure. Advantageously, this is at least 90 bar, preferably at least 100 bar, and is therefore markedly above the critical pressure of carbon dioxide.

The carbon dioxide 1 that is compressed to the desired final pressure is cooled in the aftercoolers E5B and E6 against a suitable external medium or against itself, expanded in the valve V1 to a subcritical pressure, similar to the pressure of the carbon dioxide 9 present at the exit of the low-pressure casing C1A and fed via conduit 2 to the separator D1. The separator D1 serves, inter alia, as buffer container which compensates for inventory fluctuations due to various operating states or else refrigerant losses. The gaseous carbon dioxide 3′ arising at the top of the separator D1 is fed to the precompressed carbon dioxide 9. The liquid carbon dioxide arising in the separator D1 is withdrawn via conduit 3. A substream of the carbon dioxide is expanded in the valve V3 and, in the heat exchanger E1C, serves for cooling the hydrocarbon-rich fraction A′ that is to be liquefied before said carbon dioxide substream is fed via the conduit sections 4 and 5 to the compressor unit C1A/C1B at an intermediate pressure level.

The majority of the liquid carbon dioxide 3 is divided into two substreams 30 and 40. While the first substream 30 serves for cooling the mixed refrigerant 11 of the mixed cycle, the second substream 40 serves for cooling the hydrocarbon-rich fraction B that is to be liquefied. In the embodiment of the process according to the invention shown in FIG. 1, these cooling operations each proceed in four heat exchangers E1A-E4A and E1B-E4B, respectively.

For this purpose, the two substreams are expanded into the respective heat exchangers E1A-E4A and E1B-E4B via the conduit sections 30 and 40, 32 and 42, 34 and 44, and also 36 and 46, into each of which is arranged an expansion valve a-h. The resultant gaseous carbon dioxide is withdrawn from the above-mentioned heat exchangers via the conduit sections 31 and 41, 33 and 43, 35 and 45, and also 37 and 47, and fed via the conduit sections 5-8 back to the compressor unit C1A/C1B at a suitable pressure level.

The mixed refrigerant of the mixed cycle 10 that is compressed by the compressor or the compressor unit C2 to the desired cycle pressure is cooled against a suitable external medium in the aftercooler E9 and then fed via conduit 11 through the heat exchangers E1A to E4A and cooled against the refrigerant of the precooling circuit. The mixed refrigerant 12 present in two phases at the exit of the heat exchanger E4A is separated in the separator D2 into a liquid fraction 13 and a gaseous fraction 16. The liquid fraction 13 is further cooled in the heat exchanger section E7 of the heat exchanger E7/E8, cold-producingly expanded in the expansion valve V4 and then again fed to the heat exchanger section E7 via conduit 14. In this heat exchanger section E7, the mixed refrigerant is completely vaporized against the hydrocarbon-rich fraction C that is to be liquefied and then fed via conduit 15 to the above-mentioned compressor C2.

The gaseous fraction of the mixed refrigerant 16 arising in the separator D2 is cooled, completely liquefied and subcooled in the heat exchanger sections E7 and E8, then cold-producingly expanded in the valve V5 and again fed via conduit 17 to the heat exchanger E7/E8 at the cold end thereof. Therein, the mixed refrigerant is completely vaporized against the hydrocarbon-rich fraction C that is to be liquefied and subcooled, and then likewise fed to the compressor C2 via conduit 15.

The inventory of liquid hydrocarbons in the mixed cycle is restricted substantially to the separator D2, the conduits 13, 14 and 17 between the separator D2 and the heat exchanger section E7, and also the liquids situated in the heat exchanger sections E7 and E8. Owing to the considerable reduction of the inventory of liquid hydrocarbons in the mixed cycle and the avoidance of higher or C₃₊ hydrocarbons categorized as particularly hazardous, the liquefaction system can be implemented in a more compact and thus cheaper form without reductions in safety.

In a development of the process according to the invention for liquefying a hydrocarbon-rich fraction, it is proposed that the temperature(s) of the hydrocarbon-rich fraction that is to be liquefied, of the carbon dioxide and/or of the mixed refrigerant is or are to be adjusted in such a manner that the drive powers of the compressors of the precooling circuit and of the compressor or compressors of the mixed cycle differ by a maximum of 10%, preferably by a maximum of 5%. By way of this advantageous procedure, the drives GT required for operating the compressors or compressor units C1A/C1B and C2 can be identical.

According to a further advantageous embodiment of the process according to the invention for liquefying a hydrocarbon-rich fraction, the mixed refrigerant circulating in the mixed cycle is compressed to a pressure above its critical pressure. Unwanted distribution problems of the gas and liquid phases can be avoided thereby, which can occur, in particular in a floating system (FLNG), owing to swell.

On account of the above-described advantageous procedure, the separator D2 shown in FIG. 1 is unnecessary, since the mixed refrigerant is not in two phases, but one phase, at the exit of the heat exchanger E4A.

FIG. 2 shows an embodiment of the process according to the invention in which the mixed refrigerant 10′/11′ circulating in the mixed cycle is compressed by the compressor C2 to a pressure above the critical pressure thereof. Cooling of the mixed refrigerant of the mixed cycle and pretreatment and cooling of the hydrocarbon-rich fraction that is to be liquefied are not shown in FIG. 2. These process steps run identically to those explained with reference to FIG. 1.

The mixed refrigerant 12′ cooled against the refrigerant of the precooling circuit can now be fed directly to the heat exchanger E10 and be cooled therein against itself. The heat exchanger E10 in this case replaces the heat exchanger E7/E8 shown in FIG. 1. The cooled mixed refrigerant is cold-producingly expanded in the expansion valve V6 and fed via conduit 18 back to the heat exchanger E10 at the cold end thereof. Therein, the mixed refrigerant is completely vaporized against the hydrocarbon-rich fraction C that is to be liquefied and subcooled and then fed via conduit 19 to the above-mentioned compressor C2. At the cold end of the heat exchanger E10, the liquefied and subcooled hydrocarbon-rich fraction D′ is withdrawn.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding German patent application DE 10 2012 017 653.8, filed Sep. 6, 2012, are incorporated by reference herein. 

1. A process for liquefying a hydrocarbon-rich fraction, comprising: cooling the hydrocarbon-rich fraction that is to be liquefied (A-B) in indirect heat exchange (E1B-E4B) against a multistage precooling circuit (1-9, 30-37, 40-47), wherein the refrigerant of the precooling circuit is at least 95% by volume carbon dioxide, and liquefying and subcooling the cooled hydrocarbon-rich fraction (C) is in indirect heat exchange (E7, E8, E10) against a mixed cycle (10-19), wherein the mixed refrigerant of the mixed cycle comprises exclusively component(s) selected from nitrogen, methane and ethane.
 2. The process according to claim 1, wherein said hydrocarbon-rich fraction is natural gas.
 3. The process according to claim 1, wherein the carbon dioxide circulating in the precooling circuit (1-9, 30-37, 40-47) is compressed in two separate compressor casings (C1A, C1B), a low-pressure casing (C1A) and a high-pressure casing (C1B), wherein the exit pressure of the low-pressure casing (C1A) is below the critical pressure of carbon dioxide.
 4. The process according to claim 2, wherein the exit pressure of the high-pressure casing (C1B) is operated at a final pressure of at least 90 bar.
 5. The process according to claim 2, wherein the exit pressure of the high-pressure casing (C1B) is operated at a final pressure of at least 100 bar.
 6. The process according to claim 1, wherein the mixed refrigerant circulating in the mixed cycle (10-19) is compressed to a pressure above the critical pressure thereof (C2).
 7. The process according to claim 1, wherein the temperature(s) of the hydrocarbon-rich fraction that is to be liquefied, of the carbon dioxide and/or of the mixed refrigerant is/are adjusted in such a manner that the drive powers of the compressors (C1A, C1B) of the precooling circuit (1-9, 30-37, 40-47) and of the compressor or compressors (C2) of the mixed cycle differ by a maximum of 10%.
 8. The process according to claim 7, wherein the temperature(s) of the hydrocarbon-rich fraction that is to be liquefied, of the carbon dioxide and/or of the mixed refrigerant is/are adjusted in such a manner that the drive powers of the compressors (C1A, C1B) of the precooling circuit (1-9, 30-37, 40-47) and of the compressor or compressors (C2) of the mixed cycle differ by a maximum of 5%.
 9. The process according to claim 1, wherein the hydrocarbon-rich fraction that is to be liquefied, before the cooling and liquefaction thereof, is subjected to an adsorptive water removal (Y), and wherein the liquefied hydrocarbon-rich fraction (D, D′) is expanded (V2) and the resulting gaseous fraction is compressed to the pressure of the hydrocarbon-rich fraction that is to be liquefied (A, A′) and fed thereto, wherein the compressed gaseous fraction, before the feeding thereof, serves for regenerating the dryer or dryers of said adsorptive water removal (Y).
 10. The process according to claim 1, wherein the refrigerant of the precooling circuit is at least 99% by volume carbon dioxide.
 11. The process according to claim 1, wherein higher hydrocarbons, and optionally hydrocarbons at risk of freezing, are removed from the precooled hydrocarbon-rich fraction that is to be liquefied (C). 